<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .nav  li{
            
            float:left;
        }
    </style>
</head>
<body>
<ul class="nav" id="nav">
    <li>a</li>
    <li>b</li>
    <li>c</li>
</ul>
<div class="content" id="content">
    <div>aa</div>
    <div style="display: none">bb</div>
    <div style="display: none;">cc</div>
</div>
<script>
    function $(id){
        return document.getElementById(id);
    }
</script>
</body>
</html>